The present invention relates to a method of producing an oxide dielectric element, and a memory and a semiconductor device using the element.
One type of recent semiconductor memory is a ROM (Read Only Memory) which makes use of a non-volatile characteristic allowing retention of data even in the OFF-state of the power supply to the memory. The ROM, however, has problems in that the number of re-writing operations is largely limited, the re-writing speed is low, and the like. Another type of recent semiconductor memory is a RAM (Random Access Memory) having the advantage of enabling re-writing of data at a high speed. An oxide dielectric substance is used as a material of a storing capacitor, which is a basic component of the RAM. Oxide dielectric substances can be classified into a high dielectric substance having a high dielectric constant and a ferroelectric substance having a hysteresis of polarization. In particular, a DRAM using a high dielectric substance and a non-volatile RAM using a ferroelectric substance are known. First, the non-volatile PAM using a ferroelectric substance will be described. Such a non-volatile RAM is advantageous in that a non-volatile characteristic is obtained by making use of the hysteresis effect of the ferroelectric substance; the number of re-writing operations possible is as large as 1010 to 1012; and the re-writing speed is in xcexcs ({fraction (1/1,000,000)} sec) or faster in comparison with other types of memory, and therefore, the non-volatile RAM is expected to become the future ideal memory. Developments have been made to further enhance the capacity, the non-volatile characteristic and the re-writing speed of the non-volatile RAM. The non-volatile RAM, however, has a large problem in that film fatigue occurs with an increase in the number of re-writing operations, whereby the spontaneous polarization (Pr) property of the ferroelectric substance deteriorates. To enhance the capacity and durability of the non-volatile RAM, it is known to adopt (1) a ferroelectric material having a large spontaneous polarization (Pr) and (2) a ferroelectric material which is fatigue-free. As a ferroelectric material having a large spontaneous polarization (Pr) and which is fatigue-free, an oxide having a perovskite structure is extensively available. In particular, there is a Bi-layered ferroelectric substance, SrBi2Ta2O9, having a crystal structure in which a plurality of simple lattices of the perovskite structure are layered. This material has a crystal anisotropy allowing the spontaneous polarization Pr to be directed only in a direction perpendicular to the c-axis, and the material has a good film fatigue characteristic, although the Pr value is not necessarily large. Examples of devices using such a material are disclosed in WO93/12542 (PCT/US92/10627) and Japanese Patent Laid-open No. Hei 5-24994.
Meanwhile, a DRAM using a high dielectric substance has ridden on the wave of development requiring a large capacity, for example, 16 Mbits, or even 64 Mbits, along with the advancements in high density and high integration techniques. To meet such a requirement, there has been a strong demand for making the geometries of a circuit component of the DRAM finer, particularly with respect to the geometries of a capacitor for storing information. To achieve the finer-geometries of the capacitor, attempts have been made to make a film of a dielectric material thin, to adopt a material having a high dielectric constant, and to assemble the structure having top and bottom electrodes and a dielectric substance not in two dimensions, but in three dimensions. For example, a high dielectric substance, BST [(Ba/Sr)TiO3], having a crystal structure composed of a simple lattice of the perovskite structure is a material having a dielectric constant (∈) larger than that of a conventional high dielectric substance, SiO2/Si3N4. An example using such a high dielectric substance has been reported in IEDM (International Electron Device Meeting) Tech. Dig.: 823, 1991.
The present invention relates to a method of producing an oxide dielectric element, and a memory and a semiconductor device using the same. In particular, the present invention is applicable to high dielectric elements, such as a DRAM, which makes use of a high dielectric constant and a low leakage current density, and a non-volatile RAM, which makes use of a high spontaneous polarization and a low coercive field; and a memory and a semiconductor device using the high dielectric element or the ferroelectric element.
Ferroelectric thin films and high dielectric thin films have been required to be heated up to high temperatures, for example, about 650xc2x0 C. for a thin film of Pb(Zr/Ti)O3, about 600xc2x0 C. for a thin film of (Ba/Sr)TiO3, and about 800xc2x0 C. for a thin film of SrBi2Ta2O9. That is to say, in the formation of a thin film of the above material having a perovskite type crystal structure, the material must be heated up to a temperature of 600xc2x0 C. or higher for promoting crystallization. The heat treatment at such a high temperature, however, causes various problems. For example, in the formation of a film by a vapor deposition process, a bottom electrode is exposed to an oxidizing atmosphere at a high temperature at the initial stage of the film formation, and thus is susceptible of being peeled off. Furthermore, when a film of SrBi2Ta2O9 is formed at a high temperature of 800xc2x0 C. as conventional, Bi is evaporated to cause a deviation of the film composition, so that the starting content of Bi is required to be excessive. As a result, after the film formation at a high temperature, the excessive Bi exists as an irregular phase containing Bi in a large amount at grain boundaries of the ferroelectric layer, which causes degradation of the withstand voltage characteristic, and further, a transient layer is formed by diffusion of elements at the interplane between the ferroelectric thin film and each of the top and bottom electrodes, which reduces the spontaneous polarization (Pr) to thereby degrade the original characteristics of the ferroelectric element, increases the coercive field (Ec), and causes the film fatigue. For this reason, the number of re-writing operations performed by reversing an electric field is largely limited. Further, the heat treatment at a high temperature creates problems such as (a) the formation of the reaction layer reduces the dielectric constant and spontaneous polarization, and (b) the growth of crystal grain increases the leakage current density. This leads to an increased operational voltage, thus making it difficult to achieve high integration of the element.
The present invention has been made on the basis of the above knowledge, and an object of the present invention is to provide a method of producing an oxide dielectric element having good characteristics, particularly, a ferroelectric element having a high spontaneous polarization and a low coercive field, or a high dielectric element having a high dielectric constant and a good withstand voltage characteristic; and to provide a memory and a semiconductor device using the above oxide dielectric element.
According to one feature of the present invention, there is provided a method of producing an oxide dielectric element, particularly, a ferroelectric element having a high spontaneous polarization and a low coercive field or a high dielectric element having a high dielectric constant and a good withstand voltage characteristic, characterized in that a ferroelectric thin film as a main part of the ferroelectric element or a high dielectric thin film as a main part of the high dielectric element is formed in a low oxygen concentration atmosphere at a temperature of 650xc2x0 C. or less for the ferroelectric thin film or 600xc2x0 C. or less for the high dielectric thin film. In this case, to maximize the occupied ratio of a perovskite structure in the entire crystal phase in the thin film and hence to enhance the electric characteristics of the element, the concentration of oxygen in the low oxygen concentration atmosphere may be preferably set in a range which is greater than 0.1% and less than 5.0%.
According to another feature of the present invention, there is provided a method of producing an oxide dielectric element, characterized in that the low oxygen concentration atmosphere is made variable by adjusting the mixing ratio of oxygen to inert gas, and the heat treatment is carried out at atmospheric pressure. With this configuration, the production method can be made very simple.
According to a further feature of the present invention, there is provided a method of producing an oxide dielectric element, characterized in that the ferroelectric thin film or high dielectric thin film formed in accordance with the above-described production method is re-heated in an activated oxygen atmosphere of O3, N2O, radical oxygen or the like. With this configuration, the quality of the ferroelectric or high dielectric thin film can be enhanced.
According to the present invention, the ferroelectric thin film is characterized in that it is expressed by a chemical structural formula of (AO)2+(BCO)2xe2x88x92 where A is one kind of element selected from a group consisting of Bi, Tl, Hg, Pb, Sb and As; B is at least one kind of element selected from a group consisting of Pb, Ca, Sr, Ba and rare earth elements; and C is at least one kind of element selected from a group consisting of Ti, Nb, Ta, W, Mo, Fe, Co and Cr; or expressed by a chemical structural formula of (Pb/A) (Zr/Ti)O3 where A is one kind of element selected from a group consisting of La, Ba and Nb.
The high dielectric thin film is characterized in that it is expressed by a chemical structural formula of (Ba/Sr)TiO3.
The high dielectric thin film obtained according to the present invention is characterized in that it has a dielectric constant larger than that of Ta2O5 conventionally used.
Each of the top and bottom electrode materials used in accordance with the present invention is characterized in that it consists of at least one kind of metal selected from a group consisting of Pt, Au, Al, Ni, Cr, Ti, Mo and W; at least one kind of conductive oxide of a single element selected from a group consisting of Ti, V, Eu, Cr, Mo, W, Ph, Os, Ir, Pt, Re, Ru and Sn; or at least one kind of conductive oxide having a perovskite structure selected from a group consisting of ReO3, SrReO3, BaReO3, LaTiO3, SrVO3, CaCrO3, SrCrO3, SrFeO3, La1xe2x88x92xSrxCoO3 (0 less than x less than 0.5), LaNiO3, CaRuO3, SrRuO3, SrTiO3 and BaPbO3. In the case of using a conductive oxide of a single element or a conductive oxide having a perovskite structure, the oxide is characterized in that it has a resistivity of 1 mxcexa9xc2x7cm or less for ensuring the function of an electrode material.
According to the present invention, the method of producing the ferroelectric thin film or high dielectric thin film is characterized in that the thin film is formed by a sputtering process, a Pulsed Laser deposition process or a MOCVD (Metal Organic Chemical Vapor Deposition) process in an atmosphere of a mixed gas of oxygen and an inert gas. The thin film may be formed by a spin coating process or a dip coating process using a metal alkoxide or an organic acid salt as a starting material in an atmosphere of a mixed gas of oxygen and an inert gas under normal pressure.
In the method of producing the ferroelectric thin film or high dielectric thin film according to the present invention, the re-heating treatment is performed by a sputtering process, a Pulsed Laser deposition process or a MOCVD (Metal Organic Chemical Vapor Deposition) process in ECR-oxygen plasma. The re-heating treatment may be performed by a spin coating process or a dip coating process using a metal alkoxide or an organic acid salt as a starting material by irradiation with light in an ultraviolet region.